In recent years, various game systems in which characters are displayed in a simulated 3D space generated on a monitor screen have been spread. Some of such game systems are known to simulate car race, skiing, surfing, motor-boating, snow boarding, skate boarding, etc.
In such games in which the characters move, the presence given by the games largely depend on how realistically changes in character images and background images resulting from the movement of the characters are expressed.
In a game system for such a game in which a character is moved in a simulated 3D space, a better presence can be given if a shadow to be thrown on the outer surface of an object such as a character by a light from a light source blocked by a mountain, a building or the like is created (shadowing). As a result, a game system having a rich ingenuity can be realized.
A conventional shadow creating method is such that an area is so set in a simulated 3D space as to correspond to a position and a size of a mountain, a building or the like that causes shadowing, and a propagating direction of rays from a light source, and if a reference point of an object such as a character is located in this area, a shadow is created on the entire outer surface of the object. It should be noted that a shadow model (shadow volume) is a polygon-model representation of such an area in the simulated 3D space as to create a shadow on the outer surface of an object when the object is located in this area.
However, according to the above method, even if a shadow is actually thrown on part of the object, i.e. part of the object is included in the shadow model, shadowing is either applied to the entire object or not applied at all. Therefore, no sufficient presence can be given.